CP-8841 fast busy and drops registration

[adam.hardy]

Hi everyone,

I've got 8841 phones at one of my customer offices that are not able to make or receive calls. The phones will receive IP addresses and register in CM but when called or attempting to call out will fail with either silence or a fast busy. I'll then see it drop registration in CM and after a few minutes it will come back. Other offices on the same call manager in different regions are working just fine. 

[Georgios Fotiadis]

You should narrow down the problem:

- Does registration flap for all phones at that site?

- When a phone is registered, can it dial another phone at the same site (check partitions/CSSs/DPs accordingly)?

[adam.hardy]

I got results back from that customer after I posted:

Existing SCCP phones do not flap or drop or behave in anyway like the 8841 SIP phones are

8841 phones are unable to dial in building and vice versa. When they try they get the same behaviour of a fast busy or silence and the phone(s) re-register.

[adam.hardy]

Additional details we've found:
Status message view shows TCP Timeouts, not shown at other offices
Debug view shows Reason 10 and 18 for reason for out of service. Details on what those mean I found elsewhere:
10 -- TCPtimedOut - The TCP connection to the Cisco Unified Communication Manager experienced a timeout error
18 -- TCPclosedConnectHighPriorityUcm - The device closed the TCP connection in order to reconnect to a higher priority Cisco Unified CM

[adam.hardy · Accepted solution]

Resolved: Was simple MTU again. Base MTU was set to 1300 but no adjust-mss. Once we implemented that it ran stable. No users on site now to verify but I'll post again if different.

[asafayan1]

MTU where? On the L2 switch the phone is connected to ?

[adam.hardy]

On the GRE tunnel leaving the site router to the next hop. The phone was defaulting to a higher value than the circuit was capable and without the mss adjust it wasn't correcting the phone.

[asafayan1]

Thanks. That doesn't apply to my circumstances then.

[kollerud]

Same story tonight. Just hung on registering, but after adjusting the correct MTU and adjust-mss and a phone reboot it all worked!

[asafayan1]

I'm happy to say that I'm 1 month away from DELETE *.* on all things Cisco IPT!  No more absurdly complex and overpriced IPT platforms to support. We will replace 12 VMs with a single instance of 3CX and our annual cost will be LESS than the annual recurring maintenance costs for Cisco. 

Cisco was good for my career from the early 90s thru 2015 - but for any SMB and smaller enterprises ( we have 6 locations and 400 staff ) - Cisco is NOT the correct IPT platform.  Do  yourself and your company a favor and ditch Cisco and investigate 3CX.