WP7504 - Can't Place Call; Bad Chip?


Hi everyone,

I’m having an issue with a WP7504 chip. Long story short, it can’t place calls no matter what I do. Some details ahead.

I have two WP7504 chips, one I obtained earlier this year and one a few months ago. The older chip works completely fine, but the newer chip is having issues (hence why I believe it’s the chip). I have the same firmware installed on both. I have tested calling through both Legato application code (written in C) and AT commands. ATDxxxxxxxxxx; (with an actual phone number, obviously) works on the older chip, but returns “ERROR” on the newer chip. For some reason I have to add the semicolon or it returns “NO CARRIER.”

I’m using Red Pocket as the carrier with T-Mobile as the network. I have ensured that the RAT is the same on both.

Could it be that the newer chip is configured for a different carrier, or is simply bad? I have tried everything I can from a software standpoint. I have even verified that the GPIO ports are set to the same values via AT commands. Nothing seems to have any effect.

Board: mangOH Red
Module: WP7504
Firmware: SWI9X15Y_07.12.14.00 r34472 CARMD-EV-FRMWR1 2017/11/29 18:24:42
Legato: 18.05.1_17ce41dbc4af6057ebf5fd9215b38e2a

Thank you very much in advance for any insight/suggestions. Please let me know if I should provide any other information.



So just to confirm you are trying to place a circuit switch (voice) call on the T mobile network in US?

Adding the ; on the end sets the unit to treat the call as a voice call rather than a data call (changes the signalling to the network which then treat it differently) where data calls are mostly not supported anymore, its only really now available on 2G.

Can you send the following commands to the unit as if it is powering up, USB enumerating and attaching to the network then there is no reason for it to be the unit (if there is something wrong with them normally it is catastrophic and it will point blank refuse to power up normally).


Do this for both the working and non working units. When you say you have made sure they are both on the same RAT have you checked to see if the working unit is initially on LTE but then falls back to 3G during the call? Unless you have a VoLTE configured unit then calls have to be made on 3G and if the unit is unable to fallback then the call will fail.





Thank you very much for your response. I apologize for not providing the requested info; I ended up not having access to the WP7504 modules for a while, plus a million other things came up.

However, we have resolved the issue. It turns out, actually, that Sierra Wireless is now manufacturing WP7504 chips that are LTE only and do not provide 3G fallback. We were unaware of this when we purchased our new batch of chips. Since then, we’ve obtained new chips which have the 3G fallback, and we can now place calls again.

Thank you very much again for your response, Mattt. Hopefully this info may help someone else out there having the same (or a similar) issue.