Cisco UC520 Call Transfer Issue

I'm working with a Cisco UC520 phone system and have been having issues with call transfers to 2 extensions. If someone calls the main number from the outside and requests to be transferred to either of those two extensions, then the caller is told the extension is invalid. However, anyone internal can call or transfer calls to those two extensions without issue.

What would cause this and how can I go about fixing it?
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Is your uc connected to ISDN or a sip trunk for external calls?
Can the 2,extensions  make outbound calls? *and what make re the phones
Are the two extensions regular or are they group. Make sure the extensions do not forward out to a mobile, but you do not have the rule allowing an incoming call to be forwarded out.

Incoming call => operator
Outgoing number <=        extension
masnrockAuthor Commented:
@arnold - Yes, they are both regular extensions. There are also some forwarding extensioms, but those have been working fine.
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Use a call trace to determine what is going on when an incoming call is forwarded to these extensions.
Do you have multiple offices that are interconnected and the two extentions are on the remote side?

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masnrockAuthor Commented:
There are two offices, but only one phone system. Second office has only 2 people in it. The two extensions in question are in the office with the UC520.

I can try another call trace late tonight so that I can strip out identifying information from the calls.
masnrockAuthor Commented:
I long forgot about this. But it turns out the UC520 was behaving weirdly because the unit itself is going bad. Tried talking the client into replacing the PBX, but they opted not to do so. They told a cloud provider that they want to use their existing phones, then complained that a Cisco vendor that could replace their system and keep their phones was too expensive.

@arnold - Calls would just randomly drop in the call traces with no sort of logic.
masnrockAuthor Commented:
Was hoping to get further, but sometimes you get surprised. PBX is bad, and client refuses to replace.
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