i guess that you need to add analog phone connected to FXS Port into your cucm cluster ...
Then you need to configure it with an extension in CME or CUCM server but using this way your analog phones wont be part of PSTN Phones and you...
Check DB Replication, a customer faced similar issue before a couple of months and i found out that it was related to DB replication, stop replication on all cluster and reset it and check the result.
is there a document saying that expressway 8.7.1 is not compatible with cucm IM & P 188.8.131.5200-6 upgrade available in CCO ?
Also, below document is talking about 184.108.40.20600-10 release but it is still not available yet
Are you using MGCP or H323 Gateway ?
If it rings the paging system need to connect the call from it's side ... do you hear normal ringing tone or fast busy?
CCIE Collaboration #50401