调试错误，在C++中检测到堆损坏

我在我的一个函数中遇到了问题，

Contact& Contact::operator+=(long long phone)
{
    if (isValidPhonenumber(phone)) {
        long long* tmp = new long long[this->amtNumbers + 1];
        for (int i = 0; i < this->amtNumbers; i++) {
            tmp[i] = this->m_pNumber[i];
        }
        tmp[amtNumbers + 1] = phone;
        delete[] this->m_pNumber;
        this->m_pNumber = tmp;
        this->amtNumbers++;
        return *this;
    }
}

我认为是tmp[amtNumbers + 1] = phone;导致了这种情况。但我不明白为什么？我似乎分配了足够的内存。也许我在this->m_pNumber = tmp;后错过了delete[] tmp;？

但即使有，

Contact& Contact::operator+=(long long phone)
{
    if (isValidPhonenumber(phone)) {
        long long* tmp = new long long[this->amtNumbers + 1];
        for (int i = 0; i < this->amtNumbers; i++) {
            tmp[i] = this->m_pNumber[i];
        }
        tmp[amtNumbers + 1] = phone;
        delete[] this->m_pNumber;
        this->m_pNumber = tmp;
        delete[] tmp;
        this->amtNumbers++;
        return *this;
    }
}

我遇到了溢出错误。想法，建议？这是数字类型，而不是字符;我不需要为空字符分配任何额外的内容。