Callum has been working with the squad in pre-season and played in all the friendlies so far.
The Wales under-19 international was released by Birmingham City in June when his contract expired. Last season he had a loan spell with National League outfit Telford United.
Manager Mark Yates said: “Callum has done well in pre-season, impressing us with both his ability and his attitude. We’re pleased to have him with us.
“He is a young goalkeeper with a big future and we are looking forward to working with him this season and him improving his game in our environment.”
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