All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The consent of the Wali (guardian) of the woman is a condition to the validity of her marriage to any man along with the presence of two just and trustworthy witnesses.
But the physical presence of the Wali to the contract or the wedding ceremony is not an obligation.
It is sufficient to know his consent at the time of the contract. This can be done through a written message from him or a telephone call between him and the person making the contract provided there is assurance that he is the one who is speaking not someone else.
If none of the above is possible, then the woman has to wait until she finds a way to prove her father's acceptance of the marriage. If she marries herself off without his consent, then her marriage is invalid as stated by the Prophet in a sound Hadeeth.
In order to make this marriage valid, the marriage contract should be renewed in the presence of the guardian or after receiving his approval as mentioned above.
Allah knows best.