On the LDAP side we currently don't have the 'mobile' (and 'mobileTelephoneNumber') attribute fully included in account objects (hence the varying schema errors you're seeing.) It's there in the rough so to speak, and not mapped to mobilePhone yet either; checkout /opt/zimbra/openldap/etc/openldap/schema/cosine.schema.
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Bug 32294 - Add mobilePhone field to GAL.
There is 'telephoneNumber', which is currently linked to 'workPhone' in GAL address books.
If you're trying to modify the phone number displayed in the GAL for someone:
zmprov ma
user@domain.com telephoneNumber 555-123-4321
If you want to modify an individual contact see 'zmmailbox help contact' for more info on modifyContactAttrs. With SOAP it's ModifyContactRequest, you could even use the zmsoap command via CLI.