Error 501 when sending to specific numbers via modem?

[norwegian]

Hello everyone,

I'm having a persistent and frustrating issue with my NotePager Pro setup. My system uses an internal modem (a Multitech USB) to send SMS messages. For the most part, it works without a hitch, but I'm getting a consistent Error 501 ('Invalid Recipient') when trying to send a message to just a few specific numbers.

I've double-checked that the numbers are correct, including the country code, and they can receive messages from other services. The modem itself seems to be functioning perfectly fine for all other recipients.

Has anyone else encountered this exact issue with a modem connection? Is there a specific modem setting or a known carrier-related problem that could cause this error for only a few valid numbers? Any ideas on how to troubleshoot this would be a huge help.
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https://ragdollhitstickman.io
Thanks!

[Tech Support]

Can you show us the full command and response transaction that gives you the 501 error? We'd like to have a look at it in full context.

Thanks!