"Forbidden" error when calling

[w60B-Spain]

Hi, I've installed one W60B + 2 W56H on my office using my Spanish ISP SIP settings. The issue I've found is that, it does register properly (apparently), and sometimes (only sometimes), it can receive and send calls, nevertheless, almost every other time, one of the call attempts says "forbidden" on the screen of the phone. Then, after 30" or 1' approximately, if I ty it again, it seems to work.

I've downloaded the logs from the W60B and I can see it full of messages like this:
<131>Aug 26 14:08:15 sua [1423.1781]: REG <3+error > [000] <AEM> code=0x04000009, rid=4, reason=-3,'wrong state'.
<131>Aug 26 14:08:15 sua [1423.1781]: REG <3+error > [000] <AEM> code=0x04000009, rid=4, reason=-3,'wrong state'.
<131>Aug 26 14:08:15 sua [1423.1781]: REG <3+error > [000] <AEM> code=0x04000009, rid=4, reason=-3,'wrong state'.
<131>Aug 26 14:08:15 sua [1423.1781]: REG <3+error > [000] <AEM> code=0x04000009, rid=4, reason=-3,'wrong state'.
<131>Aug 26 14:08:15 sua [1423.1781]: REG <3+error > [000] <AEM> code=0x04000009, rid=4, reason=-3,'wrong state'.
<131>Aug 26 14:08:15 sua [1423.1781]: REG <3+error > [000] <AEM> code=0x04000009, rid=4, reason=-3,'wrong state'.


Relevant Data:

- My provider is Movistar Spain, whose SIP parameters are well known:

Proxy/registrar: 10.31.255.134:5070
Domain/realm: telefonica.net
STUN: [empty]
Username: [#TELEPHONE-NUMBER]
Password: [#TELEPHONE-NUMBER]

- Both Handsets (w56H) as well as the Base (W60B) are on their latest available firmware today.
- Base HW version is: 77.0.0.48.0.0.0


- If I configure those parameters on a desktop application (phoner Lite), it does work flawlesly


Any help will be much appreciated,

Thanks in advance

Best